In general, surfaces of grain-oriented electrical steel sheets are provided with coatings in order to impart an insulating property, workability, rust resistance, and the like. The coating is usually composed of a ceramic underlying film primarily containing forsterite, which is formed during final annealing, and a phosphate-based over coating applied thereon. These coatings are formed at high temperatures, and have low thermal expansion coefficients. Consequently, a large difference in the thermal expansion coefficient occurs between the steel sheet and the coating before the temperature of a steel sheet is lowered to room temperature and, thereby, a tension is imparted to the steel sheet. Therefore, the coatings are effective at reducing the iron loss. It is desired that the coating has a function of imparting a maximum tension to the steel sheet.
In order to satisfy the above-described various characteristics, various over coatings have been proposed previously. For example, Japanese Examined Patent Application Publication No. 56-52117 proposes over coatings primarily containing magnesium phosphate and colloidal silica, and improved over coatings further containing chromic anhydride.
Japanese Examined Patent Application Publication No. 53-28375 proposes over coatings primarily containing aluminum phosphate, colloidal silica, and chromic anhydride.
In recent years, there has been a growing interest in environmental conservation and, thereby, demands for products not containing harmful substances, e.g., chromium and lead, have become intensified. In the field of grain-oriented electrical steel sheets as well, development of a method for forming an over coating not containing chromium has been desired. However, if chromium is not used, quality problems, e.g., significant deterioration of the hygroscopicity resistance and reduction of tension imparted to the steel sheet (therefore, the effect of improving the iron loss disappears) and the like, occur, and no addition of chromium cannot be realized in actual industrial production. Here, deterioration of the hygroscopicity resistance of the coating refers to that the coating absorbs moisture in the air, this moisture is liquefied partly and, thereby, the film thickness is decreased or a portion with no coating results, so as to deteriorate the insulating property and the rust resistance.
For the purpose of avoiding the addition of chromium, improving the hygroscopicity resistance of the coating, and furthermore, maintaining the tension imparted to the steel sheet, Japanese Examined Patent Application Publication No. 57-9631 describes a method for applying a coating treatment solution composed of colloidal silica, aluminum phosphate, boric acid, and sulfate. Further, methods based on the phosphate-colloidal silica based coating treatment solutions have been disclosed. In a method in Japanese Unexamined Patent Application Publication No. 2000-169973, a boron compound is added in place of the chromium compound. In a method in Japanese Unexamined Patent Application Publication No. 2000-169972, an oxide colloid is added. In a method in Japanese Unexamined Patent Application Publication No. 2000-178760, a metal organic acid salt is added.
Japanese Unexamined Patent Application Publication No. 7-18064 proposes a treatment solution for over coating, in which phosphoric acid and the like are added to a composite metal hydroxide including a divalent metal and a trivalent metal, as a technology for improving the tension induced by a coating (a tension imparted to a steel sheet by a tension coating) regardless of the presence or absence of chromium.
However, there are variations in effects of improving the iron loss and the hygroscopicity resistance by these methods, and in some cases, the iron loss or the hygroscopicity resistance deteriorates to a level which causes a problem. Such variations in quality is significant in a single coil as well, and become main cause of reduction in the amount of production, because a inhomogeneous portion must be eliminated by using a rewinding line, so that a large yield loss results and, in addition, an operation of the rewinding line undergoes pressure.
Thus, the above-described variations in quality have resulted from coating defects, which have been previously inevitably generated during formation on the surface of the grain-oriented electrical steel sheet having a coating not containing chromium. These coating defects may reach the underlying film.
It could therefore be advantageous to prevent the occurrence of coating defect and improve the surface coating properties even when a coating not containing chromium is applied to a grain-oriented electrical steel sheet.
It could also be advantageous to provide a grain-oriented electrical steel sheet, which is provided with chromium-less coatings and which realizes high hygroscopicity resistance and a low iron loss at the same level as those of a steel sheet provided with chromium-containing coatings, and a method for manufacturing the grain-oriented electrical steel sheet.